Electro-optic materials are those whose optical properties change in accordance with the strength of an electric field established within them. These materials make possible an electrically controlled electro-optic modulator for use in a light valve array.
One well known form of electro-optic modulator arrays are total internal reflection (TIR) modulators which can be employed in laser-based imaging systems for example. FIGS. 1A and 1B schematically show plan and side views of a conventional TIR modulator 10 comprising a member 12 which includes an electro-optic material and a plurality of electrodes 15 and 16 arranged in an interdigitated relationship on a surface 18 of member 12. Surfaces 20 and 22 are arranged to cause input radiation 25 to refract and undergo total internal reflection at surface 18.
In this typical conventional configuration, various electrodes 15 and 16 are grouped into electrode groups S1, S2, S3, S4 . . . Sn which are collectively referred to as electrode groups S. Each of the electrodes 15 in each of the groups are coupled together and driven with corresponding one of individually addressable voltages sources V1, V2, V3, V4 . . . Vn which are operated in accordance with various image data signals. To simplify interconnect and driver requirements, all electrodes 16 are interconnected to a common source (e.g. a ground potential). In this case, electrodes 16 are coupled in a serpentine fashion among all the groups S.
Upon the application of a suitable voltage by one of the voltage sources V1, V2, V3, V4 . . . Vn to a corresponding one of the electrode groups S1, S2, S3, S4 . . . Sn, an electric field is established in a portion of the of the electro-optic material referred to as a pixel region. The application of the voltage alters the refractive index of the electro-optic material, thereby changing a birefringent state of the pixel region. Under the application of the corresponding drive voltage, the arrangement of electrodes 15 and 16 in each of the electrode groups S1, S2, S3, S4 . . . Sn causes each of the electrode groups to behave in a manner similar to a diffraction grating. A birefringent state of the each of the pixel regions can therefore be changed in accordance with the selective application of various voltages by corresponding voltage sources V1, V2, V3, V4 . . . Vn. For example, in this case when no voltage is applied to a particular electrode group S, the corresponding pixel region assumes a first birefringent state in which output radiation 27 is emitted from surface 22 and is directed by one or more lenses (not shown) towards a surface of a recording media (also not shown) to form an image pixel thereon. In the case when a suitable voltage is applied to a particular electrode group S, the corresponding pixel region assumes a second birefringent state in which output radiation 27 is emitted from surface 22 in a diffracted form which can be blocked by an obstruction such as an aperture to not form an image pixel.
Various image features are formed on a recording media by combining image pixels into arrangements representative of the image features. It is a common desire to form high quality images with reduced levels of artifacts. In particular, the visual quality of the formed image features is typically dependant on the visual characteristics of the formed image pixels themselves. For example, one important characteristic is the contrast between an image feature and surrounding regions of the recording media. Poor contrast can lead to the formation of various image features whose edges lack sharpness or are otherwise poorly defined. Another important characteristic is the accurate placement of the image pixels on the recording media.
The conventional method of driving the arrangement of electrodes 15 and 16 as previously described can lead to various problems which can adversely impact a desired visual characteristic of the final image. For example, the sharpness of feature edges can suffer or an undesired deflection of output radiation 27 can arise. FIG. 1C schematically shows a subset of electrode groups S1, S2, S3, and S4 driven with various voltage levels by their corresponding voltage sources as follows: (V1:V); (V2:V); (V3:0); and (V4: V). Voltage level V corresponds to a drive voltage level selected to cause substantial diffraction to be created by a pixel region whereas voltage level 0 corresponds to a voltage level (i.e. a ground potential in this case) selected to not cause substantial diffraction to be created by a pixel region. When a pixel region is made non-diffracting (e.g. the pixel region corresponding to electrode group S3) the average electric potential of the electrodes 15 and 16 of the pixel region is null. However, when a pixel region is made diffracting (e.g. the pixel regions corresponding to electrode groups S1, S2 and S4) the average electric potential of the electrodes 15 and 16 of the pixel region is approximately V/2. This creates an electric potential difference of V/2 between the average voltages of non-diffracting and diffracting regions of TIR modulator 10. This can give rise to long-range electric fields that deflect radiation that is propagated within the electro-optic material to produce a beam steering effect. Although the long-range fields can be relatively weak, they typically interact with the radiation over a longer path length than the shorter range diffraction grating fields.
One possible consequence of this deflection is that image pixels formed on the recording media can be shifted and a placement error arises. The degree of the placement error can vary in accordance with the image data which controls the selective application of the drive voltages. Another possible consequence can include an increase in the diffraction broadening of an image pixel since the output radiation 27 is deflected to one side in the pupil of the imaging system, thereby reducing the effective aperture of the system. Other possible consequences can include an increased sensitivity to aberrations in the imaging system.
There is, therefore, a need for improved TIR modulators that can mitigate beam steering effects. There is also a need for improved TIR modulators that can reduce occurrences of improperly formed image pixels.